Trench isolation stress relief

ABSTRACT

In a microelectronic device formed on a substrate 12, a pair of trenches 30, 36 connected at their intersection by trench 54 which is disposed at an obtuse angle with respect to each of the pair of trenches 30 and 36.

This is a division of application Ser. No. 08,292,588 filed on Aug. 18,1994, now U.S. Pat. No. 5,448,102 which is a File Wrapper Continuationof Ser. No. 08/083,080 filed on Jun. 24, 1993 now abandoned.

BACKGROUND

This invention relates generally to structures and techniques forfabricating and isolating microelectronic devices and, in particular, tomicroelectronic devices and processes including trench isolation withstress relief.

In the fabrication of microelectronic circuits it is necessary toisolate one or more microelectronic transistors from another. Whilethere are a number of techniques available for providing such isolation,trench isolation is often used for separation between adjacent deviceislands. Trench isolation involves etching a narrow, deep groove in thesubstrate, usually silicon. After etching the groove, the inside surfaceof the groove is usually filled with an oxide or polysilicon, or both.

In a typical trench isolation process, a layer of masking material isdeposited or grown on the substrate and etched to form openings defininga pattern of the length and width of a trench. The substrate is thenanisotropically etched to remove silicon. After etching is completed asidewall oxide is grown on the inside surface of the trench and then thetrench is filled with polysilicon by deposition and etchback. As a finalstep, an oxide cap is grown on the trench. The trench masking materialis typically removed prior to the growth of the oxide cap or prior tothe formation of the sidewall oxide.

The trench is made deep enough to penetrate epitaxial layers in thesubstrate. This penetration will, in effect, decouple parasitic bipolartransistors that are inherently formed during a CMOS process.Alternatively, the trench may be made deep enough to reach a buriedoxide in an SOI material.

As part of a quality assurance program, wafers containing trenchisolation were etched to identify any defects associated with trenchlocation. As a result of this investigation, we uncovered, in the worstsamples, unusual behavior. In certain samples, device islands wererelatively free of defects whereas certain field areas where no deviceswere formed had high defect concentration.

More specifically, we noticed that in large square islands such as bondpads that contained no diffusions, there were many defects adjacent thetrench area, in particular, near the corners of the trench. The fieldarea outside the bond pads had numerous defects sonic of which emanatefrom outside corners. In all of the cases, the defects appeared to bealigned with one of the trench lines entering a corner. Indeed, thedefects appear to be distributed along an imaginary line that would be acontinuation of the trench line.

SUMMARY OF THE INVENTION

We believe that the defects are generated by material in the trenchesthat expands at different rates from the substrate material. We alsobelieve that the stress generated by this differential expansion iscumulative along the length of the trench but small or self-cancelingacross the width of the trench. In order to overcome this problem andreduce the stresses in the trench and in the substrate, we change thematerial in the trench at the intersection of two or more trenches inorder to relieve the longitudinal stress.

One way of changing the trench material at the intersection of thetrenches is to not remove all of the substrate material and leave anisland of substrate material, preferably silicon as silicon substrate,that will absorb the longitudinal stress. Another solution to theproblem is to leave a void at a trench intersection so that the voidwill absorb the longitudinal stress. As such, the invention comprises amethod for reducing stress in the substrate as well as a novel devicehaving a trench structure that is different from trench structures ofthe prior art to the extent that a different material or a changedmaterial is provided at the intersection of the two trenches, and inparticular, at the corner intersection of two trenches. The inventionalso provides for relieving stresses in a T-type (three-way)intersection of two trenches as well as a four-way intersection whereboth trenches continue through the intersection. In another aspect ofthe invention, stresses along the length of a trench are relieved byproviding selected notches in the sidewalls of the trench in order toallow the stress to dissipate in the sidewalls along the length of thetrench thereby reducing the cumulative effect of the stress along thelength of the trench.

In one embodiment of the method of the invention, a portion of thesubstrate is removed along an axial direction to provide a first trench.Another portion of the substrate is removed along another axialdirection to provide a second trench that intersects the first trench.The trench is refilled with material that creates stress at theintersection of the two trenches. The material at the intersection ofthe trenches is changed in order to relieve the stress. In oneembodiment the change of material is provided by leaving some of thesubstrate material as an island in the trench. In another embodiment ofthe invention the refill material in the trench is changed to provide avoid at the intersection of the two trenches in order to allow the voidto absorb the stress in the trench.

The invention is preferably embodied in a substrate of silicon. Usingsilicon, islands of silicon are allowed to remain in the intersection orthe corners of trenches thereby absorbing the stresses generated by thetrench refill material. Alternatively, voids may be provided at thetrench intersections.

As mentioned above, trenches generate stresses in particular at corners.These may be either inside corners or outside corners. In the case of anoutside corner, the invention provides for expanding the volume of thecorner in order to provide extra material at the corner to absorb thelongitudinal stress. Alternatively, the corner can be broken orchamfered by providing a third trench between the two trenches enteringthe corner. This third, interconnecting trench will absorb and redirectsome of the longitudinal stresses that build up at the corner. For aninside corner longitudinal stresses are relieved by expanding the radiusof the corner into the device area to provide a peninsula of differentmaterial which will absorb the longitudinal stresses directed at thecorner.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other embodiments to the invention will be betterunderstood with reference to the following drawings:

FIG. 1 is a partial plan view of a round island surrounded by a trench.

FIG. 2 is a partial plan view of a rectangular island surrounded by atrench.

FIG. 3 is a plan view of two adjoining devices each surrounded by atrench.

FIG. 4 is a partial plan view of a T-(three-way) intersection of twotrenches.

FIG. 5 is a partial plan view of a through (four-way) intersection oftwo trenches.

FIG. 6 is a partial plan view of an elongated section of a trench.

FIG. 7 is a partial plan view of an outside corner trench.

FIGS. 8 and 9 are partial plan views of outside corners having differentembodiments of the invention.

FIG. 10 is a partial plan view of an inside corner trench.

FIG. 11 is a partial enlarged view of a portion of FIG. 10.

FIG. 12 is a partial sectional view of a trench including one embodimentof the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Turning to FIGS. 1 and 2, there ate shown schematic examples of a roundisland of substrate material 16 surrounded by an annular trench 14 in asubstrate 12. In both examples, the substrate 12 is made of silicon andthe island 16 is of the same material. However, this technique may beused with other substrate materials including germanium, galliumarsenide, and others. With reference to FIG. 2, a rectangular island 18of substrate material is shown surrounded by a trench 14 in substrate12.

In a typical trench process, the substrate 12 is suitably masked and anopening having a pattern corresponding to the width of the trench ismade in the masking layer. The exposed substrate in the areacorresponding to the trench width, i.e. 14, is removed or otherwiseetched away in a manner well known in the art. Etching is highlydirectional so that the trench forms a pair of relatively parallelsidewalls and a floor. The sidewalls are spaced apart generally the samedistance top to bottom, but in any manufacturing process, this may varysomewhat. In most modem processes, the trench is generally slightlynarrower at the bottom of the trench than at the top.

In a typical process used with silicon, the sidewalls of the trench arethen oxidized to provide a coating of silicon dioxide and the trench isrefilled with polysilicon material usually via low pressure chemicalvapor deposition. Thus, the material inside trench 14 comprises an outerlayer of silicon dioxide and an inner material of polysilicon. Duringfurther processing of the wafer or substrate 12, the material in thetrench experiences alternately heat and cooling. Since the polysiliconin the trench has different grain boundaries from the monocrystallinesubstrate 12, the material in the trench will expand and contract atrates different from the substrate 12 and the islands 16, 18. Expansionof the trench refill materials along the length of the trench gives riseto a radially directed stress S_(R) in the round island which tends tocompress the silicon outside the island while leaving a tensile stressinside. Expansion along the width of the trench also gives rise to aradially directed stress but this component is compressive both insideand outside the island. As such, the stresses inside the island tend tocancel out or give a low tensile stress. Outside the island the stresscomponents add to one another to yield a highly compressive stress.

For the rectangular island shown in FIG. 2, a similar condition arises.However, the linear expansion ΔL along the length of the trenchconcentrates a compressive stress S_(L) at the outside corners. It isbelieved that this additive compressive stress along the directionsindicated explain the stress tails found in the stress defect etchedwafers. Expansion along the width of the trench, ΔW, gives rise to acompressive stress on both sides of the trench. Because of the geometryof the island the stress may not be as reduced inside the island as forround islands. In any event, it appears that the component of stress dueto ΔW is relatively small and does not generate substantial defects.

As shown in FIG. 3, defects tend to propagate along a line 23 thatappears to be in axial alignment with the trench 14 disposed along thelower edge of device 20. Such propagation of defects 23 into a device 22is undesirable. This observation coupled with the analysis for thecauses of stress given above, led to a solution of the problem as shownin FIG. 4. There, a first trench 30 is shown intersecting a secondtrench 36. The trench 30 is patterned to diverge at or near thethree-way intersection into two branches 32, 34. The branches 32, 34diverge from each other and join the trench 36 on opposite sides of anisland 40. The island 40 is of the same material as the substrate 12,preferably, silicon. In the embodiment shown in FIG. 4, the island 40acts as a sacrificial island which absorbs the defects 23 that arepropagated along the length of the trench 30. As shown in FIG. 4, theisland 40 has a generally triangular shape.

The invention may also be used at three or four-way intersections.Turning to FIG. 5, again there are shown trenches 30, 36 which intersecteach other and pass through the intersection. As the trenches approachthe intersection, they branch off into two Y-shaped branches. Thus,branches 31-34 surround a central island 42 comprised of substratematerial. With the embodiment shown in FIG. 5 longitudinal stressesalong either trench 30 or 36 are absorbed by the island 42.

With reference to FIG. 6, there is shown a further improvement of theinvention. An axially elongated trench 30 has a device area on one sideand a field area on the other side of the trench 30. On the side of thetrench adjacent the field area the trench is provided with a pluralityof notches 38. The notches 38 project and extend the sidewall of thetrench 30 into the field area. Hence, lateral stresses that build upalong the length of trench 30 may be dissipated by the notches 38 todirect defects 23 into the field areas and away from a device area thatlies across the axis of trench 30.

FIG. 7 shows a typical corner where a first trench 30 meets a secondtrench 36 in a corner 50. As indicated, a device island is on the insideof the corner and a field region is on the outside of the corner. Suchcorner regions of the device area may also be subject to defects. Thesedefects can be reduced by expanding the corner region 50. In oneembodiment of the invention as shown in FIG. 8, the corner 52 has anexpanded depth 53 that expands into the field region. This extra depthof the corner helps absorb the stresses. In another embodiment as shownin FIG. 9, the trenches 30, 36 are connected together by another trenchportion 54 that is disposed at an obtuse angle to both of the othertrenches. As such, the corner is cut by a straight portion 54 thatintersects trenches 30, 36 at obtuse angles.

Another feature of microelectronic circuits is device islands withinside corners. One such feature is shown in FIG. 10 where the deviceisland has an internal corner region 43. As shown in FIGS. 10 and 11,the trench 30 has an extended rounded portion that defines a peninsula44 that projects into the device island and is boarded by the trench 30.This rounded portion of the trench and the peninsula 44 are sufficientto dissipate device defects that would propagate along the axial lengthof trench 30.

As mentioned above, it is not only possible to relieve axial stresses byproviding sacrificial islands but also by providing selective void areasin lieu of the sacrificial islands. A typical void area is shown in FIG.12. There, a side profile of the trench 30 is shown having spaced apartsidewalls 1, 3 and a floor portion 2. A layer of insulating material,typically silicon dioxide 4 is disposed on the interior surfaces of thetrench walls and floor, 1-3. A trench filling material 5, typicallypolysilicon, is deposited in the trench 30 by a low pressure chemicalvapor deposition process. During deposition, polysilicon 5 will depositmore heavily on the upper portions of walls 1, 3 than on the lowerportions. The result of this is that a void 6 is formed through thecenter of the trench 30 and in the middle of the polysilicon material 5.After the polysilicon is deposited the surface is planarized and aprotective coating of silicon dioxide or silicon nitride 7 is applied tothe surface of substrate 10. Where this technique is used, it isimportant that the trench void be closed and not intersect the top ofthe trench.

Having thus described the preferred embodiments of the invention, thoseskilled in the art will appreciate that there are other modifications,additions, and changes that can be made to the invention withoutdeparting from the spirit and scope as defined in the appended claims.Such modifications and changes include the adaption of the invention tosubstrates other than silicon including germanium, as well as compoundsemiconductor materials such as gallium arsenide. Moreover, while thepreferred embodiment of the invention describes changing the material atthe intersection of the trenches to include substrate material or voids,those skilled in the art will appreciate that other materials could beselectively deposited in the intersections without departing from thespirit and scope of the appended claims.

What we claim is:
 1. An integrated circuit comprising:a substrate ofsemiconductor material with device areas comprising microelectroniccircuits and microelectronic devices; first, second, and third trenches,each trench having a length extending from one end of the trench to theother end of the trench and a width defined by spaced apart side walls,each trench at least partially filled with material which generatesstress and propagates defects at least in a direction corresponding tothe length of the trench; the first and second trenches extending alongintersecting paths toward a corner intersection and terminating atrespective ends prior to intersecting with each other and, the thirdtrench joining said first and second trenches at the cornerintersection, the third trench having one end terminating at the end ofthe first trench and the other end terminating at the end of the secondtrench, said third trench extending between the first and secondtrenches and said third trench being generally disposed at an obtuseangle with respect to each of the first and second trenches forrelieving the stress at the corner intersection of the first and secondtrenches.
 2. An integrated circuit comprising:a substrate ofsemiconductor material with device areas comprising microelectroniccircuits and microelectronic devices; two trenches, each trench having alength extending from one end of the trench to the other end of thetrench and a width defined by spaced apart side walls, each trench atleast partially filled with material which generates stress andpropagates defects at least in a direction corresponding to the lengthof the trench; said two trenches extending along intersecting paths anddefining a corner intersection where the paths of the two trenches meet;and, wherein the corner intersection comprises a rounded portion thatpartially circumscribes a peninsula of substrate material, saidpeninsula disposed at the intersection of both paths of the trenches andsaid peninsula comprising substrate material being without circuits ordevices for absorbing the propagated defects, relieving the stress atthe corner intersection of the two trenches and for preventing damage tothe device areas.
 3. An integrated circuit comprising a substrate ofsemiconductor material with device areas comprising microelectroniccircuits and microelectronic devices;two or more trenches, each trenchhaving a length extending from one end of the trench to the other end ofthe trench and a width defined by spaced apart side walls, each trenchat least partially filled with material which generates stress andpropagate defects at least in a direction corresponding to the length ofthe trench; at least two of said trenches extending along intersectingpaths and defining an intersection where the paths of the two trenchesmeet; and, wherein at least one of said two trenches is elongated alongan axial direction and one or more notches disposed in at least one ofthe side walls of said at least one of said two trenches to relievestress in the trench.